In particular, the present invention relates to a flowmeter for monitoring the flow of a liquid, gas, or multi-phase fluid through a conduit or pipe, particularly, but not exclusively, for use in downhole oilfield applications.
It is known to measure the flow rate of fluid flowing through a conduit or pipe in a downhole oilfield environment using flowmeters based on the Venturi principle which have throat restriction portions. A Venturi flowmeter is disclosed in U.K. Patent 2186981B in which a constant diameter section of pipe called a gradiomanometer merges smoothly with a converging section of pipe to act as a Venturi. Applicant's own U.K. Patent No. 2261519B discloses a hydrocarbon flow rate monitor which measures the flow of fluid through a production pipe by using a drill string during drill string testing (DST) of the production pipe. Measurements of pressure are taken from various points along the pipe from which fluid density and ultimately flow rate of hydrocarbons in the production pipe can be calculated. A modification of this flowmeter is disclosed in applicant's International Patent Publication No. WO 94/16295 where the Venturi flowmeter is provided with pressure measurement stations on either side of the flow restrictor to provide increased flexibility of use and accuracy of flow rate measurement.
Venturi flowmeters with throat restrictions including those mentioned above generally work well in practice but suffer from the main disadvantage of bore restriction which, in many cases, prevents tools being run through the flowmeter. These tools also require pressure sensors to be provided in the wall of the tubing where failure of a sensor can lead to abandonment of the flowmeter.
An alternative Venturi flowmeter is disclosed in the applicant's PCT Publication No. WO 97/25594, in which a flow restrictor insert is located centrally in the bore of a conduit or pipe, the flow restrictor insert being arranged so as to create an annular flow path around the insert. A remote pressure sensor downstream is used in conjunction with one of the pressure sensors at the Venturi, so as to be able to calculate fluid density ρ from the pressure difference as disclosed.
The flowmeter of WO 97/25594 suffers from the same disadvantage of bore restriction as the throat restrictor type. Wireline retrievable versions of both types can be used but these still require sensors in the tubing wall with the associated problems of sensor failure. One embodiment of the flowmeter in WO 97/25594 provides sensors in the insert. When the insert is retrievable this provides an advantage over other embodiments in the event of sensor failure.
However, the prior art flowmeters, including those disclosed in of WO 97/25594, suffer from the significant disadvantage that the accuracy of the calculated flow rates is highly dependent upon the spacing between the outer surface of the flow restrictor insert and the inner surface of the conduit or pipe in which the insert is located. This spacing is difficult to determine accurately in the downhole oilfield environment and, indeed, in other such similar applications where the insert is located in a conduit because the conduit is typically a well casing or production tubing, the surface of which tends to become coated over time by drilling materials, oil and the like adhering to the inner surface. This causes variation in tubing diameter which, in turn, causes variation in the accuracy of the pressure measurements and therefore upon the accuracy of the flow rate calculations.
Furthermore, when it is desired to measure the flow rates in conduits or pipes of different internal diameters, it is necessary to adjust the diameter of the flow restrictor insert to achieve the required spacing between the outer surface of the insert and the inner surface of the conduit or pipe. This is a time consuming process, typically requiring component parts of the insert to be changed, for example, by removing the leading portion of the insert which is shaped to create an annular flow path around the insert, and replacing this with a portion of different shape. The leading portion comprises an entry cone, a cylindrical body attached to the entry cone, and an exit cone at the trailing end of the body, attached to the remaining portion of the insert. Thus replacement of the leading portion is a complex and time-consuming procedure and is therefore expensive, requiring removal of the entry cone, the cylindrical body and the exit cone, followed by the installation of replacement parts of different outside diameter, depending on the diameter of the conduit or pipe receiving the insert.
It is amongst the objects of the present invention to obviate or mitigate at least one of the disadvantages of known inventions.